In certain known types of nuclear fission reactors, particularly research reactors, it is known to employ nuclear fuel-containing plate structures to provide the fissionable nuclear fuel, such as uranium or uranium compounds, enriched with uranium 235. In such known plate structures, a fissionable fuel material or "meat" is sandwiched between aluminum cladding plates which afford structural support and containment for the fission products.
In some cases, a homogeneous uranium-aluminum alloy has been used for the "meat". This material has excellent behavior under neutron irradiation, in that the material does not swell up or otherwise cause trouble, but this material is limited to a very low uranium density of approximately 1.3 grams of uranium per cubic meter. Because of the low uranium density, it has generally been necessary to use uranium having a high percentage of enrichment with uranium 235. The high enrichment percentage has the disadvantage that the uranium can readily be reprocessed for use in nuclear weapons.
More recently, dispersion fuels have been used to obtain higher uranium densities in the "meat". To produce such dispersion fuels, fine powders of aluminum and uranium-bearing compounds have been mixed together and pressed to form the "meat". The uranium densities thus achieved depend on the choice of the uranium-bearing compound and on its volumetric fraction in the fuel "meat". The largest uranium density achieved in commercially produced dispersion fuels has been about 1.7 grams of uranium per cubic meter. Work is underway to produce improved dispersion fuels having higher uranium densities.
The low uranium densities, mentioned above, give rise to the problem that is generally necessary to employ uranium having a high percentage of enrichment with uranium 235, so that the uranium can readily be reprocessed for use in nuclear weapons. It is desirable to reduce the enrichment percentage below about 20% to deter the reprocessing of the uranium for use in nuclear weapons.